Stack apparatus to control furnace gas exit velocity

ABSTRACT

An exhaust stack for a combustion zone of a furnace includes a main stack having a first chamber therein in communication with the combustion zone, and a main damper disposed for movement within the first chamber to be positioned to provide a cross-section of the first chamber through which combustion products from the combustion zone may flow, and a secondary stack having a second chamber therein in communication with the first chamber and being disposed within the first chamber, the secondary stack having a second damper disposed for movement within the second chamber to be positioned to provide a cross-section of the second chamber through which the combustion products from the combustion zone may flow.

BACKGROUND OF THE INVENTION

The present embodiments relate to apparatus and systems used with a heater or furnace to control or regulate an exit velocity of exhaust gas removed through a stack of the furnace.

Fired heaters and furnaces often require a minimum exit velocity for products of combustion or flue gases being discharged from the exhaust stack (or chimney) into the atmosphere. Operators of the fired heater or furnace require that an exit velocity of the combustion products is sufficient to insure adequate dispersion of the flue gases into the atmosphere without adversely affecting local ground level concentrations of any controlled emission species. The flue gas temperature and flow are affected by the operating conditions of the heater or the furnace and therefore, the existing exhaust stack discharge opening for the flue gases may be of a size that is not suitable to provide the required minimum exit velocity for the particular flue gas discharged from the product being heated. Therefore, sizing of the exhaust stack discharge opening to achieve the required minimum exit velocity in a low flow application may result in an excessive stack exit velocity for other applications.

Certain apparatus are therefore used by furnace operators to overcome the deficiencies of known exhaust stacks. For example, a fixed-diameter permanently installed exit may be mounted to the stack which operates sufficiently as long as the variation between the applications is not too great. A fixed-diameter removable exit cone may also be used, except that such a construction has two disadvantages, i.e. the cone must be installed and removed using a crane which is a maintenance function rather than an operating function and adds time, expense and inconvenience for the equipment operators; and it is limited to a two position solution, i.e. it is not variable over a wide range of operating conditions.

In addition, injection of steam or air is used to increase volume and velocity at low flow stack conditions, although such is costly in terms of utility consumption and is therefore considered impractical for most refinery fired heater and process furnace applications. Variable stack exit orifices include mechanically variable orifices used at the stack discharge point, and such are similar to a camera lens diaphragm used to control lens apertures. However, such devices have not been commercially proven for industrial application. Finally, adjustable cones have been used to adjust the velocity and/or direction of a liquid or gas stream in rocket engines and particulate scrubbing technologies. However, these devices are frequently constructed of exotic high-alloy materials and the applications of same have never been used with fired heaters and furnaces.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present embodiments, reference may be had to the detailed description of the embodiments taken in conjunction with the following drawing Figures, of which:

FIGS. 1 and 2 show a continuously variable control apparatus embodiment for a furnace stack according to the present embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The apparatus embodiment shown in FIG. 1 enables optimization of furnace exit velocities across a wide range of operating conditions in the heater or furnace when such conditions include different flow rates and temperatures of flue gas. The apparatus can be used by furnace operators without requiring the assistance of plant maintenance personnel. The apparatus does not increase operating costs due to consumption of utilities such as steam or air and is continuously variable over a wide range of operating conditions in the heater or furnace. The components from which the apparatus is manufactured are commercially available and constructed of materials already used in other parts of known fired heaters or process furnaces. Further, the present embodiments permit use of a moderate-height exhaust stack. Therefore, it is not necessary to construct an extremely tall exhaust stack with a small fixed-diameter exit cone. For purposes herein, reference to a furnace shall also include for example a heater and a melter.

Referring to FIG. 1, the continuously variable control apparatus embodiment is shown generally at 10 mounted to a roof 12 or crown of a heater or furnace. The apparatus 10 may be mounted to the roof 12 by known processes and components in the industry. A combustion zone 11 of the furnace is shown bordered by the roof 12.

The apparatus 10 includes a first or main stack 14 or flue having a side wall 15 which may be manufactured from a carbon steel material. An interior surface 16 of the side wall 15 may be lined with a castable refractory often for example of a mixture of lumnite, haydite and vermiculite. The main stack 14 may have a diameter for example of from 2 to 8 feet (0.61 to 2.4 meters). A bottom 18 of the main stack 14, opposed to a top 19 of the stack, is shown inserted through the roof 12 of the furnace so that combustion products, such as exhaust gas 20, can be exhausted from the combustion zone 11 to the apparatus 10.

The side wall 15 defines an interior space 22 of the main stack 14. A damper 24 or louver is disposed in the space 22 proximate the bottom 18 of the main stack. The damper 24 can be manufactured from for example stainless steel, and is pivotable about an axle 26 or hub to move in a clockwise and/or counterclockwise direction as shown by the arrows 28,30. The damper 24 may be manufactured as a solid plate or disk having an outer peripheral edge which when disposed in the horizontal position with respect to a center line (CL) 21 of the main stack 14 completely closes the interior space 22 to prevent the exhaust gases 20 from continuing through the main stack 14. The center line 21 may also be referred to herein as the central longitudinal axis of the main stack 14. The axle 26 is positioned on the central axis 21. In effect, the damper 24 can pivot from a completely horizontal position through to a completely vertical position with respect to the center line 21 of the main stack 14 to vary the amount of the interior space 22 open through which the gases 20 can flow to be exhausted from the main stack. Fingers 25 or projection members extend from the interior surface 16 to be contacted by the damper 24 to restrict the pivotal movement of the damper so that same does not rotate continually as a propeller.

The main stack 14 includes a lower opening 27 at the bottom 18 in communication with the combustion zone 11 of the furnace to receive the gas flow 20, and an upper opening 29 at the top 19 through which the gas flow will be exhausted from the main stack at a select velocity.

Pivotal movement of the damper 24 is done by for example an electro-pneumatic actuator (not shown). The damper 24 is shown in the fully closed position as indicated at position 32 to seal off the interior space 22. When the furnace is operating, regardless of the firing that is occurring in same, the interior space 22 annular flow area will always be open to a certain extent because the damper 24 will not be completely closed, i.e. will not be in the horizontal position, so that the gases 20 can escape the combustion zone 11.

The apparatus 10 also includes a secondary stack 34 or flue disposed at the interior space 22 of the main stack 14. The secondary stack 34 includes a side wall 36 which can be constructed from stainless steel, but does not have to be lined at its interior surface 38. The stack 34 may have a diameter of from for example 1 to 4 feet (0.3 to 1.2 meters). At least one and most likely a plurality of retaining members 40 or braces support and retain the secondary stack 34 at the interior space 22 of the main stack 14. That is, the at least one longitudinal member 40 is connected at one end to the side wall 36 of the stack 34, while an opposite end of the member is connected to the interior surface 16 of the side wall 15. The longitudinal member(s) 40 are kept as thin as possible so as not to impede or restrict the flow of the exhaust gases 20 through the interior space 22. The members 40 position the secondary stack within the interior space 22 so that a central axis of the stack 34 is symmetrical and in registration with the central axis 21 of the main stack 14.

The secondary stack 34 includes an interior space 42. A damper 44 or louver is disposed in the space 42 and is mounted to an axle 46 or a hub within the interior space 42. The damper 44 can be pivoted either clockwise or counterclockwise around the axle 46 to a completely vertical position which is parallel to the central axis of the secondary stack, or a horizontal position which is transverse to the centerline 21, as shown by the arrows 48,50. The axle 46 is positioned on the central axis 21. Pivotal movement of the damper 44 is done by for example an electro-pneumatic actuator (not shown). The damper 44 can be manufactured from for example stainless steel and formed in the shape of a solid disc or plate. Fingers 45 or projection members extend from the side wall 36 to be contacted by the damper 44 to restrict the pivotal movement of the damper so that same does not rotate continuously as a propeller.

The secondary stack 34 is provided with a lower opening 52 which is at and in communication with the interior space 22 of the main stack 14; and an upper opening 54 which is substantially in registration with the upper opening 29 of the main stack 14.

During operation, the exhaust gas 20 including the products of combustion are exhausted from the furnace to the exhaust apparatus 10. Depending upon the firing rate of the furnace, such will determine the amount of opening of the interior spaces 22,42 by the dampers 24,44, respectively. As shown in FIG. 1 by way of example only, the damper 24 is pivoted approximately 30 degrees) (30° from the horizontal so that the gas flow 56 proceeds upward through the main stack 14 toward the secondary stack 34 and the upper opening 29. If the firing rate in the furnace provides combustion products that have to be exhausted at a higher specific velocity, such may require that the damper 24 is opened for example to greater than 30 degrees) (30° from the horizontal to permit the gas flow 56 to continue through the interior space 22 of the main stack 14. It is possible that the velocity is such that the damper 44 must also be pivoted open to a certain extent to provide for gas flow 58 to flow through the interior space 42 and out through the upper opening 54 of the secondary stack 34. In this arrangement, the exhaust gas 20 originating at the combustion zone 11 will be exhausted from the interior space 22 of the main stack 14, and the interior space 42 of the secondary stack 34, to be collectively exhausted from the upper openings 29,54.

One embodiment of the apparatus 10 includes the damper 44 operable to be in either a fully opened or a fully closed position. This would provide a simpler constructed embodiment. The damper 44 would only be moved into the fully open position if the firing rate of the furnace provided combustion products that necessitated an accelerated velocity of the gas flow 56 from the interior space 22. Similarly, if the combustion products did not require to be exhausted at an increased specific velocity, the damper 44 can be fully closed to rest against the fingers 45, which would cause the gas flow 56 to move only through the interior space 22 and be exhausted through the upper opening 29. Such an embodiment would prevent the dampers 22,44 from independently searching for the correct position with respect to each other, as the movement of each damper will impact the opening position of the other damper.

Alternatively, a controller 60 can actuate the dampers 24,44 to be moved independent of each other, as shown in FIG. 2. The dampers 24,44 or the electropneumatic actuators for same of the apparatus 10 can be connected or wired 62,64 to the controller 60, which in turn senses and monitors a firing rate of the furnace to determine positioning of the dampers depending upon the minimum exhaust gas velocity that must be maintained to comply with local ordinances. Therefore, the exhaust apparatus 10 embodiments include those wherein the dampers 24,44 move independently of each other and where said dampers move concurrently with each other, and where the damper 44 is moved into either an open or closed position depending upon the position of the damper 24.

If the firing rate in the furnace is reduced to the minimum firing rate, then the secondary stack 34 will be closed, i.e. the damper 44 will be pivoted to the horizontal position so that the damper contacts or rests upon the fingers 45, wherein the damper is disposed transverse to the centerline 21 which will prevent the gas flow 56 from proceeding through the interior space 42 of the secondary stack.

The damper 44 of the secondary stack 34 is pivoted to a vertical position, i.e. parallel position with respect to the centerline 21, during the maximum firing rate of the furnace. When the furnace is at the minimum firing rate, the damper 44 is pivoted to the horizontal position to completely close off the interior space 42 of the secondary stack 34.

The gas flows 56,58 from the respective stacks 14,34 are exhausted to the atmosphere external to the apparatus 10.

The dampers 24,44 are described herein for pivotal movement. It will be understood however that movement of the dampers 24,44 can be otherwise such as by sliding movement into position or hinged movement, wherein each of the dampers is mechanically hinged at an edge thereof and then moved (opened and/or closed) to a select position within its respective interior space 22,42.

It will be understood that the embodiments described herein are merely exemplary, and that one skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention as described and claimed herein. Further, all embodiments disclosed are not necessarily in the alternative, as various embodiments of the invention may be combined to provide the desired result. 

What is claimed is:
 1. An exhaust stack for a combustion zone of a furnace, comprising a main stack having a first chamber therein in communication with the combustion zone, and a main damper disposed for movement within the first chamber to be positioned to provide a cross-section of the first chamber through which combustion products from the combustion zone may flow, and a secondary stack having a second chamber therein in communication with the first chamber and being disposed within the first chamber, the secondary stack having a second damper disposed for movement within the second chamber to be positioned to provide a cross-section of the second chamber through which the combustion products from the combustion zone may flow.
 2. The exhaust stack of claim 1, wherein the main stack and the secondary stack have a common centerline such that the main stack is concentrically arranged around the secondary stack.
 3. The exhaust stack of claim 1, wherein the main and secondary dampers are constructed and arranged to move independently of each other.
 4. The exhaust stack of claim 1, wherein the main stack further comprises an inner surface lined with a castable, heat-resistant refractory.
 5. The exhaust stack of claim 1, wherein the main stack further comprises a first inner surface defining the first chamber, and at least one first member projecting from the first inner surface into the first chamber in registration with a first line of movement of the main damper and upon which said main damper may rest.
 6. The exhaust stack of claim 1, wherein the main damper comprises a first plate having a first area substantially similar in size to a first cross-sectional area of the main chamber, and the second damper comprises a second plate having a second area substantially similar in size to a second cross-sectional area of the second chamber.
 7. The exhaust stack of claim 1, further comprising a first axle connected to the main damper for pivoting the main damper, and a second axle connected to the second damper for pivoting the second damper.
 8. The exhaust stack of claim 1, wherein the main stack comprises an upper main opening in communication with the first chamber, and the secondary stack comprises an upper secondary opening in communication with the second chamber and substantially in registration with the upper main opening.
 9. The exhaust stack of claim 1, wherein the main stack comprises a lower main opening opposed to the upper main opening and in communication with the first chamber, and the secondary stack comprises a lower secondary opening opposed to the upper secondary opening and in communication with the second chamber, the lower secondary opening disposed in the main chamber spaced apart from the lower main opening.
 10. The exhaust stack of claim 1, wherein the main damper and the secondary damper are each formed from stainless steel.
 11. The exhaust stack of claim 1, further comprising at least one brace extending from an inner surface of the main stack into the first chamber to an outer surface of the secondary stack for supporting the secondary stack within the first chamber.
 12. The exhaust stack of claim 1, wherein the secondary stack further comprises a second inner surface defining the second chamber, and at least one second member projecting from the second inner surface into the second chamber in registration with a second line of movement of the second damper and upon which said second damper may rest.
 13. The exhaust stack of claim 1, wherein the movement for the main and second dampers is pivotal.
 14. The exhaust stack of claim 1, wherein each of the main and secondary dampers are pivotal through 180° of the movement. 